1. Field of the Invention
This invention relates to detecting condensation trails behind an aircraft, and more particularly to a rear-looking system and a method for distinguishing contrails from other atmospheric phenomena occurring behind an aircraft.
2. Discussion of Prior Art
Jet engine exhaust condensation trails ("contrails") have been studied from a number of standpoints. For example, Robert G. Knollenberg has reported on various studies performed using an aircraft which was controlled so that its engines intentionally produced contrails. The aircraft was then flown into the contrails so that a forward-looking optical-array spectrometer could sense the distribution of ice crystals in the contrails. The spectrometer output was used to determine whether jet aircraft traffic increases or decreases water abundance at sub-tropopause levels. Because onboard technicians were available to detect contrail formation, and the aircraft could change course to fly through the visually-detectable contrails, the pilot in these studies was not faced with the problem of automatically (non-visually) and quickly determining whether the aircraft was causing contrails to be formed.
Through these and other studies, many aspects of contrails are known. However, for a given aircraft operating at a particular time, it is still difficult to control the engines so that they never produce contrails. Therefore, there is still a need to detect when an aircraft engine is producing a contrail, and in particular, to automatically detect such a contrail from the aircraft itself without relying on actual visual observations as in the experimental studies conducted by Knollenberg.
Systems which have been used for cloud and aerosol studies include random modulation continuous wave laser radar ("lidar"). While these systems have generally been ground-based and directed into the sky for studying clouds and aerosols, it was reported in Applied Optics, Vol. 22, No. 9, in May 1983, that such systems are suitable for airborne use. However, these and other lidar systems with which applicants are familiar (e.g., Applied Optics, January 1986, Vol. 25, No. 1; and Applied Optics, April 1990, Vol. 29, No. 10) have not been directed to distinguishing clouds from contrails, and the manner in which such systems would be mounted on the aircraft has not been specified. Further, to provide in an airborne system the altitude ranging features of a ground-based lidar system, the outgoing lidar signals of the system of the Applied Optics articles would be directed upwardly from the aircraft through the layers of the atmosphere. Based on the disclosed use of such lidar system, there would be no reason for the outgoing lidar signal to be directed other than upwardly from the aircraft.
Other atmospheric analysis systems, such as Knollenberg's noted above, have been forwardly directed, with probes being spaced to allow the contrail particles to flow through a sensing area as the plane flies through the contrail. Since the Knollenberg sensor must be in the contrail for measurement purposes, whereas the system disclosed in the Applied Optics articles is a device for making observations at long distance, the "fly through" system of Knollenberg does not suggest mounting the normally upwardly oriented lidar systems in a horizontal position to look forwardly for cloud observation.
Neither the Knollenberg nor the Applied Optics articles teach how a cloud observation device could distinguish a cloud from a contrail when the contrail is in the cloud. As to Knollenberg, it appears clear that Knollenberg would not even be faced with such problem because the goal of the studies was to analyze only the contrail. Thus, the pilot would not even fly the aircraft into a contrail which was in a cloud. As to the Applied Optics articles, even if one were to use a lidar in a forward-looking manner, there would be no assurance that differences in amplitude of return signals would be an indication of a contrail. For example, in FIG. 7 of the Applied Optics article, January, 1986, Vol.25, No. 1, aerosol and one cloud are shown having a higher amplitude return signal than a more distant cloud, indicating that such is determined by various factors.
From another aspect, if the Knollenberg or the Applied Optics airborne systems were used to look ahead through horizontally spaced cloud formations which contain contrails, the aircraft would move toward the contrail and the clouds, causing all of the returns from the clouds and the contrail to appear at ranges which vary with changes in the distance from the aircraft to the clouds and the contrail. Based on these articles, all targets which are looked at in the forward direction (as in Knollenberg) would become closer to the aircraft as the aircraft approaches them. The fact that all targets, such as clouds or contrails, would all become closer to the aircraft as it flies toward them would cause the location of all of the clouds (as indicated by the "range" scale in FIG. 7) to be shown uniformly closer to the aircraft. Thus, in this situation, there would not be any signal from the clouds or the contrail which always stays at the same range from the aircraft.
Neither the Knollenberg nor the Applied Optics articles discuss making changes in the direction of the output from the transmitter. Thus, these articles do not appreciate that there would be a difference between (1) a signal scattered back toward the aircraft from the front portion of a cloud behind the aircraft and (2) a signal scattered back toward the aircraft from the front portion of a contrail which forms behind the engines of the aircraft. In particular, the articles do not appreciate that the front portion of a contrail is generally located within the same range aft of the aircraft given stable atmospheric conditions. Therefore, the articles do not appreciate that a rearwardly directed signal which is scattered back toward the aircraft by the contrail will have a peak within that range.
These articles also do not appreciate any need to avoid detection of the contrail detecting system itself. Further, one goal of the lidars disclosed in the Applied Optics articles is to be able to detect phenomena at a range of many kilometers from the detector. Therefore, the 1986 Applied Optics article discloses that the instrument operates at 780 nm to provide weak absorption by water vapor, and thus provide an ability to obtain data from long range.
Reducing false alarms has also been a problem in airborne systems. Ward U.S. Pat. No. 4,834,531 for a Dead Reckoning Optoelectronic Intelligent Docking System granted May 30, 1989, is directed to avoiding false alarms in a satellite docking system. False alarms are avoided by continuing a target acquisition scan until a predetermined number of consecutive returns are detected.